The disclosed invention is directed generally to microwave voltage controlled oscillators, and is directed more particularly to a tunable network for a microwave voltage controlled oscillator that provides for tuning of a desired frequency sensitivity.
A microwave voltage controlled oscillator (VCO) is a circuit comprising microwave components for providing an output signal whose frequency is controlled by a control voltage. Microwave VCOs are utilized in radio frequency (RF) applications such as radar, microwave communications systems, and electronic warfare systems.
The tuning bandwidth of a VCO, i.e., the range over which the output frequency can be varied and also called the frequency sensitivity of the VCO, is typically different for different applications and requirements.
A known technique for controlling tuning bandwidth of a microwave VCO that includes a varactor diode in a tuning network is to include a variable resistor voltage divider between the diode and control voltage input which comprises the tuning port. However, the resistor voltage divider acting with the parasitic and RF by-pass capacitance of the VCO circuit reduces the modulation bandwidth attainable by the VCO. That is, the rate of change in varactor voltage is limited by the RC time constant of the resistor divider and capacitance of the circuit.
Tuning bandwidth can also be changed by adding a modulation amplifier with adjustable gain to the tuning port. However, the amplifier often introduces unacceptable phase noise to the oscillator output.